There is known a hologram recording technique of recording information to be recorded, as an interference pattern, into a holographic recording medium (hereinafter merely referred to as a “recording medium”). One method is to spatial-modulate light from a light source in accordance with the information to be recorded, to thereby generate object light, and irradiate the recording medium with the object light and reference light. The object light and the reference light form the interference pattern on the recording medium, and the interference pattern is recorded into a recording layer of the recording medium. On the other hand, in reproduction, the interference pattern recorded in the recording medium is irradiated only with the reference light, and detection light from the recording medium is detected by a 2-dimensional (2D) sensor, to thereby reproduce the information.
As in the information recording on an optical disc or the like, even in the hologram recording, the object light and the reference light are focused on the recording medium, by using an objective lens or the like. Therefore, in order to accurately perform the information recording or reproduction, so-called tracking servo control is required, which is to control the position of the objective lens with respect to the recording medium. A patent document 1 describes an example of the tracking servo control of drive-controlling the objective lens on the basis of a tracking error signal, which is generated by irradiating laser light onto a guide groove formed in advance on the recording medium, on a hologram recording/reproducing apparatus. Incidentally, there are patent documents 2 and 3, for example, as other documents which describe the hologram recording.    Patent document 1: Japanese Patent Application Laid Open NO. 2001-273650    Patent document 2: Japanese Patent Application Laid Open NO. 2002-63733    Patent document 3: Japanese Patent Application Laid Open NO. 2000-122012